It's apple for anything with wings in my WSM. For a pellet cooker, I've come to the conclusion that BP is right. Blind folded and gun to my head I wouldn't be able to tell the difference unless mesquite pellets were used.
I think a big part of the difference is in the smell of the smoke. But remember, a big part of your taste comes from smell.
I think Scooter's on to something with WMS vs. Pellet cookers because I think cooking temp plays a big role. I can tell a lot more difference in foods that have been cold smoked such as cheese and sausage. The smoke is allowed to penetrate those foods a lot more. I have been experimenting with Sugar Maple for smoked sausages some lately and am really liking the difference it gives.
Some of the woods I can't tell as much difference. But I can tell a huge difference between Apple & say Hickory or Black Walnut, even in the pellet cookers.
The only way I know to explain what I taste is nut woods leave more mellow warm almost salty overtones, whereas apple leaves a slightly more pungent campfire smell with a hint of sweetness. Like you guys mentioned, Mesquite really stands out as very strong, slightly bitter, almost peppery. Sugar Maple is a very light sweet smoke. Pecan is like hickory (because it is a type of hickory) but lighter in flavor.
The other difference I see is in the color different woods impart to your food. The fruit woods definitely give a darker patina especially Apple & Cherry. I think that's another part of why those two woods are so popular for poultry.
Oak is a technically a nut wood, but it doesn't really have a strong specific smoke flavor like the various hickory's or walnuts. It's like a general purpose smoke that works with anything. Its a very clean burning wood.